Ok, I changed the XF86Config-4 file like you suggested (and made a backup in case), but unfortunately none of the configurations (ExplorerPS/2 - "ZAxisMapping" "6 7", ExplorerPS/2 - "ZAxisMapping" "8 9", or IMPS/2 - "ZAxisMapping" "8 9") fixed the roller mouse problem. In fact, each of these options disabled the roller mouse altogether.

What I've got is a Logitech Mx500 7 Button Optical Mouse: Link Here.

Well try
Option "ZAxisMapping" "7 8"
one time with IMPS protocol and the other time with the Explorer protocol

and if this does not help then the last one:
with (change both options: "Buttons" and "ZAxisMapping")
Option "Buttons" "3"
Option "ZAxisMapping" "4 5"

Can you attach the mouse directly to USB ?

I can attach the mouse directly, but I'd have to remove my scanner or zip drive, and I'd rather not. Is there a reason USB would be more effective in this instance than the serial port?

To be honest I doubt that you are using the serial port. I think it's an USB to PS/2 converter.
Well, wenn you go through USB your system will use different frivers.

You may also consider to update your system, actually the kernel.

EXCELLENT!! The rollerball now works!

I tried both the 7 button configurations and found that neither of them effected the mouse (Explorer/2 actually threw me into Blackscreen mode), but the 3 button configuration, where "ZAxisMapping" "4 5", worked. My other buttons are, of course, all screwed up. The line-jump-up button asks if I want to search the internet for some term, the line-jump-down button, while jumping down a few lines at a time, highlights text as does so. The top-thumb button basically repeats my right-click button, while the buttom-thumb does the same thing as the line-jump-up button - asking me to search the internet again. And the Windows button does, expectedly, nothing. So, it's still buggy, but at least the rollerball is now working.

Once again, you saved my Linux, Vlad-A. Thank you!

What should I write instead of: Protocol "PS/2" when I'm using a usb? I'm currently have "auto" but the scrolling doesn't work... I have wrote: Option "Buttons" "5" och ZAxisMapping "4 5" but with no success. My mouse is /dev/mouse (default, I think)

Thx!

Edit: It's a MS optical usb mouse with 2 buttons and a scroll. The scroll currently works as a regular left button....